Fix incorrect length check in DNSName when extracting qtype or qclass

In `DNSName::packetParser()`, the length check might have been incorrect
when the caller asked for the `qtype` and/or the `qclass` to be extracted.
The `pos + labellen + 2 > end` check was wrong because `pos` might have already
been incremented by `labellen`. There are 3 ways to exit the main loop:

* `labellen` is 0, the most common case, and in that case the check is valid
* `pos >= end`, meaning that `pos + labellen + 2 > end` will be true regardless
of the value of `labellen` since it cannot be negative
* if `uncompress` is set and a compressed label is found, the main loop is
broken out of, and `labellen` still holds a now irrelevant, possibly non-zero value
corresponding to the first byte of the compressed label length & ~0xc0.

In that last case, if the compressed label points to a position > 255 the check
is wrong and might have rejected a valid packet.
A quick look throught the code didn't show any place where we request decompression
and ask for `qtype` and/or `qclass` in a response, but I might have missed one.

Reported by Houssam El Hajoui (thanks!).

(cherry picked from commit 7b9c052)

Author: rgacogne

pdns/dnsname.cc

@@ -141,15 +141,15 @@ void DNSName::packetParser(const char* qpos, int len, int offset, bool uncompres
   if(consumed)
     *consumed = pos - opos - offset;
   if(qtype) {
-    if (pos + labellen + 2 > end) {
-      throw std::range_error("Trying to read qtype past the end of the buffer ("+std::to_string((pos - opos) + labellen + 2)+ " > "+std::to_string(len)+")");
+    if (pos + 2 > end) {
+      throw std::range_error("Trying to read qtype past the end of the buffer ("+std::to_string((pos - opos) + 2)+ " > "+std::to_string(len)+")");
     }
     *qtype=(*(const unsigned char*)pos)*256 + *((const unsigned char*)pos+1);
   }
   pos+=2;
   if(qclass) {
-    if (pos + labellen + 2 > end) {
-      throw std::range_error("Trying to read qclass past the end of the buffer ("+std::to_string((pos - opos) + labellen + 2)+ " > "+std::to_string(len)+")");
+    if (pos + 2 > end) {
+      throw std::range_error("Trying to read qclass past the end of the buffer ("+std::to_string((pos - opos) + 2)+ " > "+std::to_string(len)+")");
     }
     *qclass=(*(const unsigned char*)pos)*256 + *((const unsigned char*)pos+1);
   }

pdns/test-dnsname_cc.cc

@@ -610,6 +610,45 @@ BOOST_AUTO_TEST_CASE(test_compression) { // Compression test
   BOOST_CHECK_EQUAL(dn.toString(), "www.example.com.");
 }
 
+BOOST_AUTO_TEST_CASE(test_compression_qtype_qclass) { // Compression test with QClass and QType extraction
+
+  uint16_t qtype = 0;
+  uint16_t qclass = 0;
+
+  {
+    string name("\x03""com\x00""\x07""example\xc0""\x00""\x03""www\xc0""\x05""\x00""\x01""\x00""\x01", 25);
+    DNSName dn(name.c_str(), name.size(), 15, true, &qtype, &qclass);
+    BOOST_CHECK_EQUAL(dn.toString(), "www.example.com.");
+    BOOST_CHECK_EQUAL(qtype, 1);
+    BOOST_CHECK_EQUAL(qclass, 1);
+  }
+
+  {
+    /* same but this time we are one byte short for the qclass */
+    string name("\x03""com\x00""\x07""example\xc0""\x00""\x03""www\xc0""\x05""\x00""\x01""\x00""", 24);
+    BOOST_CHECK_THROW(DNSName dn(name.c_str(), name.size(), 15, true, &qtype, &qclass), std::range_error);
+  }
+
+  {
+    /* this time with a compression pointer such as (labellen << 8) != 0, see #4718 */
+    string name("\x03""com\x00""\x07""example\xc1""\x00""\x03""www\xc1""\x05""\x00""\x01""\x00""\x01", 25);
+    name.insert(0, 256, '0');
+
+    DNSName dn(name.c_str(), name.size(), 271, true, &qtype, &qclass);
+    BOOST_CHECK_EQUAL(dn.toString(), "www.example.com.");
+    BOOST_CHECK_EQUAL(qtype, 1);
+    BOOST_CHECK_EQUAL(qclass, 1);
+  }
+
+  {
+    /* same but this time we are one byte short for the qclass */
+    string name("\x03""com\x00""\x07""example\xc1""\x00""\x03""www\xc1""\x05""\x00""\x01""\x00", 24);
+    name.insert(0, 256, '0');
+
+    BOOST_CHECK_THROW(DNSName dn(name.c_str(), name.size(), 271, true, &qtype, &qclass), std::range_error);;
+  }
+}
+
 BOOST_AUTO_TEST_CASE(test_pointer_pointer_root) { // Pointer to pointer to root
 
   string name("\x00""\xc0""\x00""\x03""com\xc0""\x01",9);